LOS GATOS — If you look up the word “fad” in Wikipedia, the single summarizing illustration of that flash-in-the-pan phenomenon is a picture of the Pet Rock. In the whole history of mankind and housebroken igneous gravel, it’s for sure nobody ever made a million bucks selling rocks in a box until Gary Dahl did it during four frenzied months in 1975.

Dahl, who died March 23 in Oregon at age 78, came up with the idea, probably over a shot of Johnnie Walker Red at a Los Gatos bar called the Grog & Sirloin popular with layabouts and jazz musicians.

Some of his drinking buddies had been complaining about cleaning up after their pets, when Dahl said he avoided any such unpleasantness with his pet rock. That barroom boast altered the course of his life, making the rocks in his future more important than the ones in his glass.

An advertising man, Dahl persuaded the president of a San Jose ad agency over a “Mad Men” lunch to help him launch the Pet Rock at precisely the moment America — weary of Watergate and Vietnam — decided it needed a good, stupid laugh. Dahl gave it to them with an owner’s manual that reflected his sense of the absurd, noting, “If, when you remove the rock from its box it appears to be excited, place it on some old newspapers. The rock will know what the paper is for and will require no further instruction.”

Before the craze ended, abruptly, more than 1.5 million Pet Rocks had been sold at department stores such as Macy’s and Neiman Marcus — which rolled giant boulders into its Dallas flagship and covered them with Pet Rocks. At its peak just before Christmas that year, the stoned startup shipped 50,000 wrapped rocks.

Time magazine called Dahl’s mineral miracle the marketing coup of the decade. It was all of that, and when you think about it, probably a good deal more. Dahl recruited the Grog’s bartender as his operations chief, began importing pebbles at a penny apiece from Mexico, and put hundreds of mentally disabled children to work on his unlikely assembly line.

Then Dahl would repair to his ad agency office, or to one of the Los Gatos liquor lairs that frequently served as his idea incubators, for some R&D on the rocks. When it was all over, Dahl used the money he’d made to finance what he facetiously described as an “eight-year vacation,” and opened his own bar, which he called Carry Nation’s, after the temperance movement radical famous for attacking rum rooms with a hatchet. “He’d always had a fantasy of owning his own bar,” says his wife, Marguerite. “His mother was a cocktail waitress.”

“He did a lot of other things in his life, but everything was focused on the Pet Rock, so he didn’t like to talk about it,” says Bruce Riddell, who became a friend of Dahl and his wife after admiring their dog, a Shiba that was part of the living, breathing menagerie of pets the couple kept during their four decades together. That was an unspoken irony of the Pet Rock saga: Gary Dahl loved animals, and was rarely without a raucous retinue of chickens, goats, dogs and cats.

During their final years in Los Gatos, the Dahls lived with an eccentric cat named Strauss. “Strauss ruled the house,” says Scott Rice, a former San Jose State University English professor, who was a frequent guest, “holding the other cats in disdain and treating his alleged owners as staff, according to Gary.”

Rice is the longtime administrator of San Jose State’s annual Bulwer-Lytton Fiction Contest, which challenges contestants to concoct a dreadful first line of prose. Dahl won the grand prize in 2000 with an entry that managed to incorporate his interest in rock formations and drinking establishments.

“The heather-encrusted Headlands, veiled in fog as thick as smoke in a crowded pub, hunched precariously over the moor,” Dahl began, trailing off horribly from there. Later, he published a book called “Advertising for Dummies.”

For the rest of his life, Dahl was besieged by every easy-money mountebank who ever dreamed of getting a whole lot of something for nothing, which was how they saw the Pet Rock.

“He said some of the ideas that people came up with were just lunacy,” Riddell says. “He could be a little gruff” when he was approached for advice. Dahl had tried several follow-up schemes of his own, including a sand-breeding kit that would supposedly allow you to grow your own beach. “It didn’t go over,” Dahl conceded after forming Rock Bottom Productions in 1976. Callers to the outfit’s office were told, “You’ve reached Rock Bottom.”

He ran a boat brokerage in Santa Cruz Harbor for a while, became commodore of the Los Gatos Yacht Club — an organization founded in yet another Los Gatos watering hole — and when the novelty of his novelty item wore off, donated 100,000 Pet Rocks that he’d overproduced to the U.S. Marines’ annual Toys for Tots campaign. But by Christmas 1976, children unwrapping a rock from Santa were probably less amused than a year earlier. “The fad has run out,” Dahl said.

“He never was able to replicate it, though he tried with a number of things,” recalls John Heagerty, a partner at Coakley Heagerty, the firm that took a fateful flyer on the Pet Rock, and later sued him to recover a share of the profits.

Before his death, Dahl suffered from chronic pulmonary disease. His widow will scatter his ashes over San Francisco Bay in May. There will be no headstone.